Interpretive Summary: Veterinary antibiotics are widely administrated to animals in modern confined animal feeding operations to treat infectious disease, protect animal health, and improve feed efficiency. Research has found that veterinary antibiotics enter the environment and, through leaching and runoff, may contaminate water bodies. Tetracyclines are a major veterinary antibiotic family with activities against bacterial diseases. Hydrolysis and photo-degradation are major dissipation routes for antibiotics in surface and ground water. Temperature and pH were found to significantly affect the degradation rate and complete dissipation of oxytetracycline was found to be very rapid in water illuminated by sunlight. Further, the effect of calcium ions on the degradation of oxytetracycline was determined. Photo-degradation was found to be the most dominant dissipation pathway and the results of this study provide important information that will help land-use manages to understand the various factors affecting hormone fate in surface and ground water.

Technical Abstract:
Oxytetracycline ((2Z,4S,4aR,5S,5aR,6S,12aS)-2-(amino-hydroxy-methylidene)-4-dimethylamino-5,6,10,11,12a-pentahydroxy-6-methyl-4,4a,5,5a-tetrahydrotetracene-1,3,12-trione) is a member of tetracycline antibiotics family and is widely administered to farm animals for the purpose of therapeutical treatment and health protection. Increasing attention has been paid to the environmental fate of oxytetracycline and other veterinary antibiotics with the occurrence of these antibiotics in the environment. The hydrolysis and photolysis degradation of oxytetracycline was investigated in this study. Oxytetracycline hydrolysis was found to obey the first-order model and similar rate constant values ranging from 0.094 ± 0.001 to 0.106 ± 0.003 day-1 were obtained at different initial concentration ranging from 10 to 230 µM. Solution pH and temperature were shown to have remarked effects on oxytetracycline hydrolysis. The hydrolysis in pH neutral solution appeared to be much faster than in both acidic and alkaline solutions. Oxytetracycline half-life decreased from 1.2 × 102 to 0.15 day with the increasing temperature from 4 ± 0.8 to 60 ± 1'C. The presence of Ca2+ made oxytetracycline hydrolytic degradation kinetics deviate from the simple first-order model to the availability-adjusted first-order model and greatly slowed down the hydrolysis. Oxytetracycline photolysis was found to be very fast with a degradation rate constant at 3.61 ± 0.06 day-1, which is comparable to that of hydrolysis at 60'C. The presence of Ca2+ accelerated oxytetracycline photolysis, implying that oxytetracycline become more vulnerable to sunlight irradiation after chelating with Ca2+. The photolysis may be the dominant degradation pathway of oxytetracycline in shallow transparent water environment.